A base station of a wireless communication system transmits a signal with a large difference between average power and peak power. In recent years, a digital transmitter that converts a baseband signal to be transmitted into a digital transmission signal including a radio frequency band component and amplifies the signal has been studied as a technique for improving efficiency of a transmission amplifier used for a transmitter such as the base station as mentioned above. As an amplifier to be applied, for example, a switch-mode amplifier such as a D-class amplifier and an S-class amplifier has been studied.
A switch-mode amplifier amplifies power, assuming that a pulse waveform signal is used as an input signal, while a pulse waveform of the input signal is maintained. The pulse waveform signal amplified in the switch-mode amplifier is output from the digital transmitter after frequency components other than a desired radio signal band are removed.
There is a demand for increasing an output of transmission power of a transmission signal or further enhancing purity and quality of the transmission signal. Synthesizing digital transmission signals is effective as a means for further enhancing the transmission power, and purity and quality of the transmission signal.
In addition, to cope with the recent rapid increase in mobile traffic and improvement in communication rate using a carrier aggregation (CA) for allowing a plurality of frequency bands to be adapted to communication at the same time, the number of frequency bands to be applied to communication has been increasing and it has been necessary for a radio transmitter and a power amplifier to be compatible with a plurality of frequency bands.
As a signal-synthesizing means for digital transmission signals in a digital transmitter, for example, as described in PTL1 of International Publication No. WO2014/042205 “TRANSMITTER, SIGNAL-SYNTHESIZING CIRCUIT, SIGNAL-SYNTHESIZING METHOD”, there has been proposed a means including a band-limiting unit that band-limits output signals from a plurality of switch-mode amplifiers, and a voltage/current source conversion unit that converts the output signals from the switch-mode amplifiers from voltage to current, the band-limiting unit and the voltage/current source conversion unit being connected to thereby synthesize signals.
For example, FIG. 7 is a block diagram illustrating an overall configuration of the transmitter described in the above-mentioned PTL1. The transmitter includes a digital baseband signal generation unit 410, a modulation circuit 420, switch-mode amplifiers 100-1 and 100-2, a signal-synthesizing circuit 200, and an antenna (load) 300. As described later, the signal-synthesizing circuit 200 includes a band-limiting unit and a voltage/current source conversion unit.
However, in the above-mentioned PTL1, NPL1 of “A Stub Tapped Branch-Line Coupler for Dual-Band Operations,” (IEEE Microwave and Wireless Components Letters, Vol. 17, Issue 2, February 2007) by H. Zhang et al., and NPL2 of “Design of a dual-band GaN Doherty amplifier,” (Microwave Radar and Wireless Communications, 2010) by P. Colantonio, a configuration using a plurality of transmission lines as illustrated in FIGS. 8A and 8B is illustrated as a specific configuration of a ¼ wavelength transmission line adaptable as a component of a signal-synthesizing circuit compatible with a single transmission frequency or two transmission frequencies. However, there is a problem that a circuit size increases as the number of transmission frequency bands increases, which makes it difficult to achieve a downsized signal-synthesizing circuit compatible with a plurality of transmission frequencies. In this regard, FIG. 8A is a block diagram illustrating a configuration example in which the voltage/current source conversion unit described in the above-mentioned PTL1 is configured to be compatible with two frequencies, and FIG. 8B is a block diagram illustrating another configuration example in which the voltage/current source conversion unit described in the above-mentioned PTL1 is configured to be compatible with two frequencies.
Further, as described in the above-mentioned PTL1, as a means for implementing a signal-synthesizing circuit to be compatible with a plurality of transmission frequencies, there is a means in which a band-limiting unit and a voltage/current source conversion unit are provided as a signal-synthesizing circuit and the voltage/current source conversion unit is provided with ¼ wavelength transmission lines as illustrated in FIGS. 8A and 8B of a synthesizing circuit unit for each of transmission frequencies f1, f2, . . . , and the ¼ wavelength transmission lines are switched by changeover switches (RF switches) 220-3, 220-4, 220-5, 220-6, . . . as illustrated in FIG. 9. FIG. 9 is a block diagram illustrating a configuration example in which the signal-synthesizing circuit described in the above-mentioned PTL1 is configured to be compatible with a plurality of frequencies. However, in the case of the signal-synthesizing circuit as illustrated in FIG. 9, when the number of syntheses of the power amplifier (PA) is represented by N and the number of transmission frequency bands is represented by M, (N×M) ¼ wavelength transmission lines are required, which causes a problem that a size of the signal-synthesizing circuit increases along with an increase in the number of syntheses and the number of bands.
Further, as a means for achieving a downsized signal-synthesizing circuit compatible with a plurality of transmission frequencies, it is possible to employ a means for allowing a passive element such as a variable capacitance and a variable inductor using an RF-MEMS (Radio Frequency Micro-Electro-Mechanical System) or the like to be adaptable to the signal-synthesizing circuit of a lumped-constant configuration illustrated in the above-mentioned PTL1. However, in general, the variable inductor has a large loss, which makes it difficult to adapt the variable inductor to a power amplifier filter and a signal-synthesizing circuit which are required to have a low Q-value (Quality factor) and high efficiency. Accordingly, in practice, an adaptable variable passive element is limited only to a capacitance. This causes a problem that, when only the variable capacitance is adapted to the signal-synthesizing circuit illustrated in the above-mentioned PTL1, an impedance characteristic of the signal-synthesizing circuit fluctuates with respect to a plurality of frequencies and output power also fluctuates in accordance with the fluctuation.